Recently, in mobile communication systems, there has been a strong demand for making mobile communication terminals smaller in size and lower in power consumption. To realize these requirements, for example, the transmission power amplifier has to be able to run on a single positive power supply, to be able to be driven by a lower voltage, and to have a higher drive efficiency. As devices currently in use for such power amplifiers, a junction type field effect transistor (JFET) a heterojunction type FET (HFET), a Schottky barrier gate field effect transistor (metal semiconductor field effect transistor: MESFET), a heterojunction type FET using a p-type gate (p-type gate HFET), etc. can be mentioned.
Among these, a p-type gate HFET has a p-n junction at the gate and thus can accept a high voltage applied to the gate. Therefore, the p-type gate HFET is operable by the single positive power supply. Further, since it has a hetero structure, it is a device showing an excellent linearity characteristic. Moreover, the threshold voltage of the FET is determined by the Al or In contents of the layers formed by epitaxial growth, the thicknesses of the layers, the carrier concentrations, and other factors determined at the time of epitaxial growth and by the diffusion depth of the P-type gate.
Generally, for mass production, device manufacturers often purchase epitaxial substrates produced by other manufacturers for producing epitaxial substrates and process the epitaxial substrates to form transistors. On the other hand, when producing p-type gate HFETs, in the epitaxial substrate manufacturers, GaAs, AlGaAs, InGaAs, and other epitaxial layers are formed on GaAs or other substrates. However, these layers generally suffer from variance in content of Al or In, variance in carrier concentration, and variance in thickness. Control of the thicknesses or carrier concentrations of the layers included in such epitaxial substrates was difficult for the device manufacturers.
FIG. 1 is a cross-sectional view of an example of the configuration of a conventional semiconductor device.
A GaAs substrate 112 serving as a semiconductor substrate is formed with a buffer layer 114, the buffer layer 114 is formed on its top surface with a channel layer 116 forming a channel of a transistor, and the channel layer 116 is formed on its top surface with an AlGaAs layer 118 as a diffusion layer. The AlGaAs layer 118 is formed with a SiN film 120 as an insulating film. Moreover, the AlGaAs layer 118 is formed with a gate electrode 124, a source electrode 121, and a drain electrode 123 insulated by the SiN film 120. The AlGaAs layer 118 of the diffusion layer formed under the gate electrode 124 is formed with a doping region 125 by selective diffusion of, for example, a p-type impurity Zn as a carrier, whereby the semiconductor device 101 is formed.
As the method of producing a conventional semiconductor device, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-188077 discloses diffusing a p-type impurity Zn in GaAs or AlGaAs by measuring the electrical characteristics after diffusion, calculating a diffusion coefficient from the characteristics, calculating the amount of diffusion to obtain the desired threshold voltage, raising the temperature of the wafer again to diffuse the impurity based on the results of calculation, and measuring the electrical characteristics after cooling the wafer to thereby control the diffusion depth.
However, the diffusion depth changes along with a change of the diffusion time, temperature or the gas flow rate, so it was not possible to focus the characteristics of semiconductor devices formed in the same substrate. Here, “focus” means, for example, to make the threshold voltage of semiconductor devices the desired value. In other words, the method of producing the conventional semiconductor device 101 suffered from the problem that when doping the p-type impurity to form the doping region 125, as shown in FIG. 2, control of the doping region 125 was difficult. Therefore, an IC (integrated circuit) produced from the center part of a wafer and an IC produced from the circumference sometimes had different threshold voltages. As a result, the threshold voltages of the semiconductor devices 101 produced from one wafer did not become uniform. Due to this, of course, some devices could not be used as ICs and the yield declined. In addition, there were the problems that the wafer temperature rose, time was required for cooling, and the TAT (turn around time) of the process was long. Therefore, a semiconductor device able to maintain the characteristics of the semiconductor device and able to be given the desired threshold voltage by a single diffusion and a method of producing the same have been desired.